1. Field of the Invention
This invention relates to a two piece piston which incorporates a high strength cast ferrous crown having a constant wall thickness together with a separate machined piston skirt made of various ferrous and non-ferrous materials; conjoined by the piston/connecting rod wrist pin.
2. Description of Related Art
Internal combustion (IC) engines have been utilized for years in stationary and mobile applications. Examples of the former include pumps, generators, oil field equipment, compressors, and the like, while examples of the latter include heavy tractors, trucks, earthmoving equipment, automobiles, marine propulsion and auxiliary uses and the like.
Recent developments to the numerous types of IC engines in the last fifteen years have demonstrated that in the diesel engine and high power gaseous fueled applications of such engines, substantial thermal efficiencies, increases in power as a ratio of engine displacement, and reductions in emission can be achieved by increasing the combustion pressure and in the case of the diesel engine, the fuel injection pressures.
These increases in mechanical and thermal efficiency have been achieved through increasing intake air pressure by a factor of several magnitudes of atmospheric pressure by the utilization of mechanical and/or turbo supercharging, by increasing diesel fuel injection pressure and with precision mechanical and/or electronic means of controlling the operation and thermal condition of the subject IC engine by the use of electronic engine management systems.
These developments have all resulted in an increase in the temperature of the combustion process in both the diesel and gaseous fuel iterations of the IC engine which has manifested itself in the form of piston top (crown) temperatures that exceed the thermal limits of known materials and applications.
Known methods of cooling such pistons by use of oil jets from beneath and temporary retention and heat rejection by captured oil delivered by such means have failed to solve the problems satisfactorily in most applications.
The makers of IC engines and parts have further sought many avenues of materials and design to solve the dual problems of material strength at elevated temperatures and acceptable material weight.
This concurrent need for thermal strength and acceptable weight is the result of the piston in an IC engine being a moving, in fact, reciprocating part that moves through the piston bore of such engines at high linear speeds in order to translate combustion pressure on the piston through connecting rod into rotational energy at the crankshaft.
In addition, the piston in its cylindrical bore has been traditionally and remains sealed between the combustion part located between the top of the moving piston and the cylinder top or head and the remainder of the engine by a multiplicity of sealing rings that are installed in circumferential groove machined into the outer diameter of the piston itself, each ring being in the form generally of a rectangular cross section that is radially cut to permit its elongation and installation in the groove in the piston.
In the most recent development of IC technology it has further been proven that the closer that the top most of the aforementioned sealing rings can be installed to the top of the piston itself, the less stagnant or residual gasses remaining from the preceding combustion event will be present and the amount of certain undesirable combustion by products including but not limited to oxides of nitrogen and monoxides of carbon will be substantially minimized by the engine in its operation.
This desire to particularly locate the topmost piston ring has by itself posed unique material and design problems that have not been satisfactorily addressed in a cost effective manner by existing designs and iterations of piston technology.
Although there have been numerous methods applied by the makers of engines and pistons to solve these multiple objectives (high strength, thermal stability, ring groove stability, production costs) none have been entirely satisfactory from either a weight or strength standpoint, or alternatively, if such a design and operational balance is approached, it is by methods and designs that are substantially more costly to produce that the prior common aluminum IC piston that has been the standard for over 60 years.
In this search for acceptable dual qualities of thermal strength and acceptable component weight, among the methods used are the following, each with its unsatisfactory characteristics noted:
1. High strength aluminum pistons:
Heat resistant alloys are costly and difficult to forge or cast, will not withstand combustion pressures and temperatures at existing engine power levels, and prematurely fail in service;
2. Cast or forged aluminum or aluminum alloy pistons with cast in place ferrous inserts for ring grooves and piston tops/combustion cavities:
Costly to manufacture and at high temperatures the remaining aluminum eventually erodes or loses necessary thermal strength;
3. One piece cast iron pistons that mimic aluminum designs:
Heavy weight and inconsistent expansion/thermal characteristics limit applications and combustion pressures due to poor weight strength ratio;
4. Two piece pistons with forged and machined ferrous crowns connected to cast/forged and machined aluminum skirts by the use of high strength elongated gudgeon/wrist pins:
Very high cost to manufacture piston crowns;
5. Forged and machined ferrous piston crowns that are joined by mechanical means or friction welding to ferrous or non-ferrous skirts with a common piston/gudgeon pin:
Very costly to manufacture, compromised thermal characteristics and unsatisfactory in long term service;
6. Forged and machined one piece ferrous skeleton piston:
In addition, since these pistons, of whatever design, do wear in service, particularly in comparison to the life of the entire engine where pistons may be replaced five or ten times in a typical engine's installed service life; thus for this reason, a substantial market has developed for pistons utilized both in the initial, typically name brand, production of the engines as well as in the aftermarket repair and rebuilding of the engines.
In consideration of the above, piston manufacturers are constantly developing new technology relative to existing designs in a search for longevity of initially installed pistons as well as those used in the rebuilt/remanufactured processes in order to lengthen the service life of a particular engine block.
Examples of these efforts include the Detroit diesel engine as set forth in U.S. Pat. No. 5,299,538; the Cummings piston as set forth in U.S. Pat. Nos. 5,144,844 and 5,339,352; the Mercedes engine as set forth in U.S. Pat. Nos. 3,363,608 and 4,413,597; and, the Caterpillar piston as set forth in U.S. Pat. No. 4,056,044.
In addition to the above, additional piston designs have been developed by various manufacturers in order to increase the initial and subsequent service life of the engine. An example is the Mack piston as set forth in U.S. Pat. No. 4,180,027.
The purpose of these various engine and piston designs is said to provide increased thermal equalization, mechanical stability, and longer service life. While they may do so, the cost of the tooling and manufacturing processes is significant, and the secondary machining operations are numerous, complicated, and costly; finally not always resulting in acceptable in service life or desired engine performance characteristics.